The formation of water soluble polyaluminosilicate microgels and their use in papermaking is known. U.S. Pat. No. 5,176,891 discloses a process for their production involving the initial formation of a polysilicic acid microgel followed by reaction of the polysilicic acid microgel with an aluminate to form the polyaluminosilicate. The use of the polyaluminosilicate microgels as improved retention and drainage agents in papermaking is also disclosed. U.S. Pat. No. 5,127,994 discloses a process for the production of paper by forming and dewatering a cellulosic fiber suspension in the presence of three compounds: an aluminum salt, a cationic polymeric retention agent and polysilicic acid.
The polyaluminosilicate microgel process disclosed in U.S. Pat. No. 5,176,891 comprises three steps, namely, (1) the acidification of a water solution of an alkali metal silicate to form polysilicic acid microgel, (2) addition of a water soluble aluminate to the polysilicic acid microgel to form the polyaluminosilicate and (3) dilution to stabilize the product against gelation. There is a necessary aging period involved following the acidification step during which the silicic acid first formed polymerizes to linear polysilicic acid and then to the microgel structure which is critical to the performance of the polyaluminosilicate products. The products are described as having a surface area of greater than 1000 square meters per gram, a surface acidity of greater than about 0.6 milliequivalents per gram and an alumina/silica mole ratio of greater than 1:100, preferably between 1:25 and 1:4.
The method of the present invention is an improvement over the process described in the '891 patent in that it combines the acidification and alumination steps. An unexpected and important benefit resulting is that the aging period required for microgel formation to occur is significantly reduced. The polyparticulate polyaluminosilicate products produced by the process of this invention have good activity as retention and drainage agent in paper making immediately on formation (no aging period) and they reach their optimum performance in significantly less time than those prepared by previous methods. Aging periods required for product formation are avoided or minimized whenever possible in paper making since they require additional or oversized equipment and are known to give rise to problems such as products of uneven quality. Any reduction in the aging period is thus an improvement in the process of paper making and in product quality.